The present invention relates to a process for preparing a lithium manganese oxide (LiMnO.sub.2) powder having an .alpha.-NaMnO.sub.2 type layered rock-salt structure. This lithium manganese oxide powder is useful as cathode materials for lithium rechargeable batteries.
At the present time, lithium rechargeable batteries are used as a power source for portable electronic/electric appliances. Lithium cobalt and nickel oxides (LiCoO.sub.2, LiNiO.sub.2, and solid solutions thereof) having an .alpha.-NaFeO.sub.2 type layered rock-salt structure have been studied and developed and put to practical use as cathode materials for the lithium rechargeable batteries. Although these cathode materials advantageously have high operating voltage and high capacity, they contain a rare metal, cobalt (Co) or nickel (Ni), and hence are expensive. This is an obstacle to the expansion of market of lithium rechargeable batteries using the cathode material (the cost of the cathode material occupies about one-third of the material cost of the battery).
Further, lithium manganese oxides, such as LiMn.sub.2 O.sub.4 and LiMnO.sub.2, have attracted attention as low-cost cathode materials of an advanced 4-volt class, and research and development thereof are being conduced. In particular, for LiMnO.sub.2, manganese has a valency of 3, whereas for LiMn.sub.2 O.sub.4, manganese has a valency of 3.5. Therefore, for LiMnO.sub.2, a higher discharge and charge capacity than that for LiMn.sub.2 O.sub.4 can be expected, and, hence, LiMnO.sub.2 is a most promising, advanced low-cost cathode material. LiMnO.sub.2 compounds known in the art are classified into two crystal phases (orthorhombic phase (.beta.-NaMnO.sub.2 type structure; hereinafter referred to as "orthorhombic LiMnO.sub.2," and a monoclinic phase having a layered rock-salt structure (.alpha.-NaMnO.sub.2 type structure; hereinafter referred to as "layered rock-salt LiMnO.sub.2 ").
However, conventional methods, that is, a method wherein a mixture of various lithium and trivalent manganese compounds is subjected to a solid phase reaction at 500 to 900.degree. C. (R. J. Gummow and M. M. Thackeray, j, Electrochem. Soc., 141[5] (1994)1178) and a method wherein the above mixture is hydrothermally treated at 150 to 300.degree. C. (M. Tabuchi, K. Ado, C. Masquelier, H. Sakaebe, H. Kobayashi, R. Kanno and 0. Nakamura, Solid State Inoics, 89, (1996)53), can provide only orthorhombic LiMnO.sub.2. In this phase, lithium can be electrochemically eliminated/inserted. Since, however, repetition of discharge and charge causes gradual transition to another crystal phase (spinel phase), the stability of discharge and charge curves with respect to discharge and charge cycles is disadvantageously low.
Therefore, the establishment of a process for preparing layered rock-salt type LiMnO.sub.2 having the same crystal structure as LiNiO.sub.2 or LiCoO.sub.2 has been urgently demanded in the art. At the present time, this compound is synthesized by ion-exchanging .alpha.-NaMnO.sub.2, synthesized by a conventional solid phase reaction in an nonaqueous solvent containing lithium ions at a temperature of 300.degree. C. or below (A. R. Armstrong and P. G. Bruce. NATURE. 381, [6]. (1996)499; F. Capitain, Pravereau and C.Delmas, Solid State Ionics, 89, (1996)53: these two documents being hereinafter referred to as "references"). In an industrial process, these methods require two stages, i.e., preparation of .alpha.-NaMnO.sub.2 and ion exchange thereof, unfavorably making it difficult to mass-produce layered rock-salt LiMnO.sub.2. Therefore, the development of an alternative novel practical process has been desired in the art.